1. Field of the Invention
This invention relates to an ambient-noise monitor and a timing control for a seismic recording system.
2. Discussion of the Prior Art
In marine seismic surveying, a ship tows through the water along a line of survey, a streamer cable containing a plurality of hydrophones and a corresponding plurality of electrical conductors. Periodically, as the ship occupies each designated recording station, a sound source is fired to generate acoustic waves. The generated acoustic waves propagate downwardly through the water and into underground rock layers. Reflected from the rock layers, the acoustic waves return to the water surface where they are detected by the hydrophones. The detected signals are transmitted to and processed by suitable recording equipment.
In relatively deep water the steamer cable is towed continuously at a desired depth, usually in the range of 20 to 40 feet. At that depth, even though the streamer cable is continuously towed at a speed of five knots or more, the towing-noise level is satisfactorily low. The streamer cable is held at a substantially constant depth by means of pressure-actuated hydrofoils or "birds". The above operational mode is defined as a continuous tow.
In very shallow water it is not practical to use the pressure-actuated hydrofoils. The streamer cable is held at a constant depth by suspending it from surface buoys or floating from a tethered weight. Under tow, the surface buoys and their associated tether lines generate an intolerable amount of noise. Accordingly when data are to be recorded by the hydrophones, the cable must lie substantially at rest. It is impractical, too time-consuming, and too uneconomical to stop the towing ship every thirty seconds or so when data are to be recorded. Hence, means are provided for first reeling in part of the cable. When a data recording is to be made, the cable is released, unreels, drifts and so becomes substantially stationary in the water while the towing ship continues on her way. At the end of the recording period, the cable is again partly reeled in and the free-wheeling cycle is repeated after the fashion of a yo-yo. This operational mode is defined as an intermittent tow.
U.S. Pat. No. 4,314,363, assigned to the assignee of this invention, discloses a complete system for operating in shallow water employing the intermittent tow mode of operation. The specification and drawings of that patent are incorporated herein by reference. There are of course some earlier primitive systems, namely U.S. Pats. No. 3,281,767 to Cryar, 3,286,225 to Huckabay et al., and 3,359,536 to Coburn. Those systems were discussed at length in the '363 reference patent.
As taught by the reference patent, the hydrophone cable is payed out for several seconds as the towing ship moves ahead. As the cable is payed out, it drifts to a standstill. A two-second delay is introduced so that the cable can settle down to provide a quiet-time window during which a recording cycle can be executed.
The timing of the sequence of events was arbitrarily chosen based upon average expected sea conditions but without regard to the actual ambient conditions existing at the time of a particular recording-cycle execution. Thus, if the recording cycle was executed too early or too late, the seismic data suffered from noise contamination. If the quiet-window period was widened to allow more leeway for executing a recording cycle, the operating efficiency of the survey crew was impaired.